152 
STURGEONS. 
low, grovel along the bottom, feeding in sboals on the 
decomposing animal and vegetable substances which are hurried 
down with the debris ot the continents drained by those rapid 
currents. Thus they are ever busied in re-converting the 
substances, which otherwise would tend to corrupt the ocean, 
into living organized matter. “These fishes are therefore duly 
weighted by a ballast of dense, dermal, osseous plates, not 
scattered at random over their surface, but regularly arranged, 
as the seaman knows how ballast should be, in orderly series 
along the middle and at the sides ot the body. The protection 
against the water-logged timber and stones hurled along their 
leeding-grounds, which the Sturgeons derive from their scale 
armour, renders needless the ossification of the cartilaginous 
case of the brain or other parts of the endoskeleton, and the 
weight of the armour requires that endoskeleton to be kept 
as light as may be compatible with its elastic property and 
other functions. The Sturgeons are further adjusted to their 
place in the liquid element, and endowed with the power of 
changing their level and rising with their defensive load to 
the surface by a large expansive air-bladder.” Protection to 
the eye is even more necessary than the other portions of 
the body, and accordingly this is provided for, in addition to 
the bony crust that surrounds and overtops it, by being deeply 
sunk in its small chamber, into which probably it still more 
deeply falls when danger tlireatens. 
How far the habits of these fishes will support the wide 
interpretation applied to their rigid armature by the above-named 
eminent philosopher, I am not prepared to decide; but there 
is another benefit, consistent with that already mentioned, 
which is derived from the presence of those plates, and which 
will be of great use to these fishes in some of the situations 
in which they must often find themselves placed. The bones 
of Sturgeons are remarkably soft, even for a race of fishes in 
which none of the bones are hard and firm; and they do not 
possess ribs, which organs aftord so strong a tulcrum for 
support to the action of muscles in bony fishes, but their place 
is well supplied by those substantial plates, which are not 
simply a covering to the surface, but dip within between the 
layers of the organs of motion, and thus enable the muscles 
of the body to exert such a degree of action as otherwise 
they would not be capable of. 
